1. Field of the Invention
The present invention relates to a pressure switch used for detecting a pressure increase in a hydraulic apparatus.
2. Discussion of Related Art
In a hydraulic power steering apparatus used in an automobile or the like, a hydraulic pump is generally driven by an engine, and operation oil discharged from the hydraulic pump is supplied to the hydraulic power steering apparatus so as to assist the driver's steering operation.
In such a hydraulic power steering apparatus, when a steering operation is carried out while the automobile or the like is stopped and therefore the engine is running at its idle rotational speed (hereinafter referred to as "stationary steering operation"), the load pressure of the hydraulic pump increases, resulting in an engine stall. In order to solve this problem, the conventional automobile or the like employs a so-called idle-up mechanism and also employs a pressure switch which detects an increase in the load pressure of the hydraulic pump while the engine is running at the idle speed so as to operate the idle-up mechanism.
FIG. 1 shows an example of such a pressure switch. In FIG. 1, numeral 1 denotes a switch housing, which is attached to a pressure introduction passage 3 formed in a pump housing 2.
In the switch housing 1 is supported a piston 4 which is movable in the axial direction. Load pressure introduced through the pressure introduction passage 3 acts on the piston 4. Disposed above the piston 4 is a terminal 5 serving as one of a pair of electric contacts. This terminal 5 is held by the holding ring 7 via a resin 6 serving as an electrical insulator.
Between the terminal 5 and the piston 4 is disposed a disk spring 8 serving as the other electric contact. When the piston 4 presses the disk spring 8, the disk spring 8 comes in contact with the terminal 5. The disk spring 8 is held between the holding ring 7 and the switch housing 1, and is grounded to a vehicle body via the switch housing 1 and the pump housing 2.
The holding ring 7 is placed into an accommodation bore 1a formed in the switch housing 1, and an end portion 1b of the switch housing 1 is deformed inward through caulking, so that the holding ring 7 is fixed to the switch housing 1, thereby positioning the disk spring 8. After the caulking, a resin 9 is charged into the opening portion of the accommodation bore 1a so as to seal the opening portion.
When the load pressure of the hydraulic pump increases due to a stationary steering operation while the engine is running at the idle speed, the increased pressure acts on the piston 4, so that the piston 4 slides against the spring force of the disk spring 8. Consequently, the piston 4 elastically deforms the disk spring 8, thereby bringing it into contact with the terminal 5. With this operation, electrical conduction is established between the pump housing 2 and the terminal 5, so that an electrical signal for idle-up is generated.
Since such a pressure switch is disposed in an engine space, the adhesive force between the end portion (caulked portion) 1b of the switch housing 1 and the resin 9 charged in the opening of the accommodation bore 1a decreases due to salt damage caused by antifreezing agents scattered on roads or due to thermal shock, resulting in formation of a gap between the resin 9 and the caulked portion 1b. In this case, moisture and the like enters the inside of the switch housing 1 through the gap and causes adverse effects on the electric contact portion between the disk spring 8 and the terminal 5.
In the conventional pressure switch, a thread is formed on the outer circumference 1c of the lower end of the switch housing 1, and the lower end of the switch housing 1 is screwed into a threaded hole 2a formed at the opening end of the pressure introduction passage 3 of the pump housing 2 of the hydraulic pump.
In order to screw the switch housing 1 into the pump housing 2, an intermediate portion 1d of the switch housing 1 has a hexagonal shape. Since the hexagonal intermediate portion 1d overlaps with the press-fit portion of the holding ring 7 and the inner circumference of the intermediate portion 1d is circular so as to accommodate the holding ring 7, the intermediate portion 1d has thin-wall portions on which stress is concentrated upon press-fitting of the holding ring 7. When the concentrated stress exceeds the breaking point of the material of the switch housing 1, the holding ring 7 becomes loose, resulting in positional shift of the disk spring 8.
This problem also occurs when the switch housing 1 is screwed into the pump housing 2. That is, when a spanner or a like tool other than a box wrench is used to screw the switch housing 1 into the pump housing 2, the intermediate portion 1d of the switch housing 1 deforms, so that the holding ring 7 becomes loose, resulting in positional shift of the disk spring 8. Even when a box wrench is used, a similar problem occurs if the switch housing 1 is tightened with a force exceeding a predetermined torque.
This causes a possibility that the pressure switch does not become ON even when a predetermined pressure acts on the pressure switch, resulting in degradation of the performance of the pressure switch.